A method for bulk polymerization of monomers having a polymerizable unsaturated bond consisting mainly of an alkyl acrylate, which comprises adding a polymerization initiator having a 10 hours half-life temperature of 41.0 C. or below to the monomers in an amount of 0.0001 to 0.5 parts by weight based on 100 parts by weight of the monomers to initiate polymerization of the monomers, allowing after the reaction initiation the temperature of the reaction mixture to reach a maximum of 100 to 140 C. by the self-exotherm of the reaction system due to the consumption of the polymerization initiator, and thereby polymerizing 15 to 50% by weight of the monomers used.